Question
A 46-year-old sheep farmer develops a painful, enlarging reddish-purple nodule on the dorsum of his index finger. He frequently handles lambs, some of which have recently had crusted lesions around their mouths. The finger lesion began as a small papule and has gradually become a raised, partly ulcerated nodule. He is otherwise well and has no fever or systemic symptoms.
What is the most likely diagnosis?
A. Herpetic whitlow
B. Orf
C. Cutaneous leishmaniasis
D. Felon
E. Sporotrichosis
Answer
Orf
Explanation
Orf (contagious ecthyma) is a zoonotic infection caused by a parapoxvirus. It primarily affects sheep and goats and is transmitted to humans through direct contact with infected animals or contaminated material.
The classic MRCP clue is:
Sheep/goat exposure + solitary nodular lesion on the hand/finger → Orf.
In humans, the lesion usually develops on the hands, fingers, or forearms, where minor skin trauma allows viral inoculation. It typically begins as a small red or red-blue papule, gradually enlarging over several weeks into a 2–3 cm nodular lesion that may become flat-topped, weeping, ulcerated, or haemorrhagic.
The patient is usually systemically well. Orf is generally self-limiting, resolving spontaneously over approximately 4–8 weeks, although lesions can sometimes persist longer.
Anthrax is an important differential in people exposed to livestock, but cutaneous anthrax classically produces a painless lesion that develops a characteristic black eschar, often with marked surrounding oedema. A painful nodular lesion following sheep or goat contact is much more characteristic of orf.
Paronychia or a felon usually produces a bacterial infection around the nail fold or fingertip pulp rather than the characteristic enlarging nodular lesion associated with animal exposure.
Summary for quick exam revision
Orf is a zoonotic viral infection caused by a parapoxvirus. It is mainly found in sheep and goats, where it causes crusted or “scabby” lesions around the mouth and nose. Humans usually acquire infection through direct contact with infected animals, particularly through small breaks in the skin. Farmers, shepherds, abattoir workers, and veterinarians are therefore typical patients. Human lesions usually occur on the hands, fingers, or forearms. The lesion initially appears as a small red or red-blue papule. It progressively enlarges into a nodular lesion that may reach around 2–3 cm. The surface may later become flat-topped, ulcerated, weeping, or haemorrhagic. Patients generally remain systemically well. Diagnosis is usually clinical when the characteristic lesion is combined with sheep or goat exposure. The major exam differential is cutaneous anthrax, which usually causes a painless black eschar with surrounding oedema. Bacterial paronychia primarily affects the nail fold and does not have the typical animal-exposure history. Orf is usually self-limiting and commonly resolves without specific antiviral treatment. The key MRCP association to remember is sheep or goat contact + nodular hand lesion = Orf.